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UT Technique | Petrochemical
Plants |
Boilers |
References
DEVELOPMENT OF NDE TECHNOLOGY
Anmol Birring, President of NDE Associates, Inc., has been involved with the
development of NDT techniques for detection of HTHA since 1981. Most of the inspections
at that time used the ultrasonic amplitude technique, the reliability of which was
always questionable. Mr. Birring worked on several projects on this subject
with JGC, Japan, Idemitsu, Japan, EPRI and Chevron. During that period, he
identified the relationship of ultrasonic backscatter with HTHA. In 1990, he was awarded US Patent,
4,890,496 for developing this technology. Since then backscatter
technique has been widely used worldwide in the petrochemical plants.
We provide inspection services worldwide for HTHA assessment.

Hydrogen
attack (a) The dark area on the Boiler Tube ID represents hydrogen
damage. (b) Hydrogen Attack in the 18 mm thick pipe sample from a failure in a Refinery de-suphurization
plant in Japan. Failure date March 31, 1982. Note the
depth of HTHA at failure is 70 percent.
RELIABILITY OF HTHA INSPECTION
The reliability of HTHA inspections depends on the skill of the
inspector. We have run into cases where inspectors missed
last stages of HTHA. Plant owners must carefully select
inspectors to perform these inspections.
One question to ask would be if the
inspector has ever found HTHA in previous inspections ? There are inspectors
who have inspected over hundred components but never reported HTHA
SCHEDULING EQUIPMENT FOR HTHA
INSPECTIONS
1. C-1/2 Mo Equipment
operating above the API 941 limits must be replaced. Piping must be
replaced immediately because of its low thickness.
2. Reactors, heat exchangers shells and other thicker equipment must be
inspected at the earliest if immediate replacement is not possible.
Ultrasonic backscatter inspection must be performed by skilled inspectors
who have a track record of detecting actual HTHA in the plant equipment, not
just inspecting for HTHA.
3. Selection of inspection locations is critical. Inspection must be
performed at the hottest locations of the equipment. HTHA susceptibility is
highly dependent on the heat of the plate and is not very well understood.
Just a small difference in heat input or treatment can make a major difference in HTHA
susceptibility. So one plate/spool may be totally free of HTHA while the
adjoining plate/spool operating at the same temp/pressure conditions in the
same vessel may have high levels of HTHA. Each and every plate of the vessel
must therefore be tested at both ends. Preferred locations for HTHA are the
intersection points of the plates/spool.
4. Any suspect areas detected by ultrasonic testing must be verified by
replication of magnetic particle testing from the inside surface.
5. After the inspection, equipment operating above the API 941 limits must
be planned for replacement. In the meantime, regular inspections must
monitor for any HTHA.
6. HTHA is dangerous. Do not take any chances. Most NDT inspectors
performing HTHA inspections miss HTHA. We have run into cases where skilled
inspectors have missed last stages of HTHA in the base metal. HTHA in the
weld HAZ is difficult to detect and can be easily missed.
REACTOR TAKEN OUT OF SERVICE in france
A case in
point is inspection of a C-0.5 Mo reactor that was tested by NDE Associates,
inc.
in 2004. The reactor at the Total refinery in Le Harve, France was placed in service in 1972
. The refinery was concerned of any damage by HTHA as the reactor was operating
above the C-steel curve. In 2004 NDE Associates inspected the reactor using the ultrasonic backscatter
technique and found the depth to be about 40 percent of the
thickness, indicating significant levels of HTHA. Follow-up replication confirmed HTHA. Further metallurgical and magnetic particle tests
from the reactor ID showed severe HTHA. Based on the results, the
refinery decided to take reactor out of service. The experience showed the
effectiveness of the inspection in removing the equipment out service and
avoiding a failure. (Note: Prior to this inspection another company in 2002 missed HTHA)
NDE Associates, Inc. has performed several HTHA inspections. Some of our clients are:
- Total Fina, Le Harve, France, 2004
- Total, Rome, Italy, 2005
- Total, Milford Haven, UK, 2007
- Chevron Oil, Richmond Refinery, CA. Conduct
follow up inspections after the April 1989 fire caused by Hydrogen Attack
failure.
- Chevron, Pascagoula, MS
- BP, Texas City, 2005
- other refineries in US, Canada
- Huntsman Chemicals, Melbourne, Australia.
- SK Oil, Korea. Conduct follow up inspections
after the failure caused by Hydrogen Attack. Failure date: May 13, 1999.
- CF (Fertilizers) Industries, Donaldsonville,
Louisiana.
- Conoco Oil Company, Billings Refinery, MT
and Denver Refinery, CO.
Background
Steel suffers from hydrogen damage or high temperature hydrogen attack
when a seepage of hydrogen reacts with metal carbides
to form methane gas. This reaction decarburizes the steel, produces micro
cracks, and lowers the toughness of the steel, but does not necessarily
cause a loss of thickness. Detection of hydrogen attack is
important to ensure the safe operation of pressure vessels and piping
that is susceptible to such damage.
MC + 4H = M (M:metal) +CH4
(methane)
The type of damage caused by hydrogen
attack depends on the source. The source of hydrogen in boiler
tubes is from a reaction of steam and steel. Hydrogen damage in
boiler tubes is, therefore, always associated with ID corrosion.
However, in chemical plant the source of the hydrogen is from the
flow stream (hydrocarbons) and, therefore, there is no
corrosion associated with hydrogen damage.

Damage caused by HTHA failure in a refinery
Petrochemical and Ammonia Plants
High temperature hydrogen attack (HTHA) in the
petrochemical and ammonia plants is caused when the hydrogen from the stream
seeps into carbon and low alloy steels at high temperatures. Hydrogen reacts
with the carbides in the steel, decarburizing the steel and forming methane gas
bubbles at the grain boundaries with no loss of thickness. The methane gas bubbles
grow with time and result in micro cracking. The combination of micro cracking
and decaburization reduces the fracture toughness of steels and lead to major
failures.
HTHA is triggered in the components operating
at high temperatures and high hydrogen partial pressure. API
941's Nelson curves provide information about the safe operating
environment for components operating in hydrogen environments (1).
Components operating in the unsafe environments are susceptible
to hydrogen attack. The concern is especially true for C-1/2 Mo steels whose
curve has been been lowered and now dropped to the same level as that carbon
steel (1). Because of this drop, some of the C-1/2 Mo components originally
operating in the safe region are now in the unsafe region of the Nelson curves.
These components should either be inspected regularly for evidence of HTHA or
replaced with a higher grade of steel.
Hydrogen Attack can occur both in the
base metal and the weld HAZ. The attack in the base metal is wide
spread and distributed uniformly. In addition to temperature and hydrogen
partial pressure, the susceptibility of attack also also depends on the microstrocture ferrite/bainite or ferrite/pearlite with quasi M23C6 carbide (2). Weld attack is very localized
and grows along the HAZ. The susceptibility of attack in the weld
HAZ depends on the heat treatment. Post-weld heat treated welds
are less susceptible to hydrogen attack compared to the welds
that are not heat treated. There have been several cases where
the depth of the attack in the HAZ is greater than the depth
in the base metal. It is therefore imperative that both the base
metal and the weld HAZ be inspected for presence of hydrogen
attack.
Some of the factors that are important for
scheduling inspections are as follows:
- operating conditions (partial pressure of hydrogen and
temperature) relative to the operating limits provided in
API 941, 1997 edition (1).
- slow cooled C-1/2 Mo steels have less resistance to
hydrogen attack than normalized steels (2).
- post-weld heat treated welds are
less susceptible to hydrogen attack compared to the welds
that are not heat treated.
A general discussion of HTHA prediction is given in
reference 5.
NDT Techniques for Petrochemical and Ammonia Plants
Base metal attack is detected by using a
combination of ultrasonic back-scatter and velocity measurements (6,7,
8). Hydrogen attack increases the ultrasonic backscatter and reduces
the ultrasonic velocity in the material. In particular, HTHA increases the ratio
of S-wave to L-wave velocities. The backscatter and velocity ratio measurements
are applied to detect hydrogen attack. The ultrasonic backscatter technique was
developed by A. S. Birring in 1989 and was first applied at the Chevron Richmond
Refinery in 1989 (7). Details of the UT techniques for HTHA
inspection are given in references 6 and 7.
Application of the ultrasonic techniques for HTHA detection requires an individual
with a good understanding of the mechanism of HTHA and how it affects the
propagation and scattering of ultrasonic waves. One has to understand that
that while hydrogen attack affects velocity-ratio, backscatter and the frequency
of the reflected signal, other material anomalies can influence these
ultrasonic parameters as well and give a false call. Ultrasonic inspection for
this application is therefore not straight forward and requires a logical test
methodology to detect HTHA.
Phased Arrays are not recommended for detection of base metal HTHA

Application of ultrasonic backscatter for detection of HTHA. The
ultrasonic backscatter is ID connected.
Weld HAZ attack is detected using the
ultrasonic shear wave technique. Since the cracking caused by
hydrogen attack is in the weld HAZ is extremely fine, the shear wave
inspection is done at a very high sensitivity.
Both the base metal and weld HAZ should
be inspected for hydrogen attack.
Further verification of attack can be
done by surface replication.
Note:
This website is solely for informational
purposes. In no event will this company be responsible for any loss or
damages resulting from any viewer's use of these materials.
References
- Steels for Hydrogen
Service at Elevated Temperatures and Pressure in Petroleum
Refineries and Petrochemical Plants, API Recommended
Practice 941, Fifth edition, American Petroleum Institute, 1997.
- T. Ishiguro,
H.Yamamoto, K. Kawano, et al, "Metallurgical Effect on Hydrogen Attack Damage
in C-½Mo
Steels," Proceedings, 1996 ASME/ICPVT Pressure Vessels and piping Conference,
21-26 July, 1996
- Hattori, K. and Aikawa, S.,
"Scheduling and Planning Inspections of C-0.5Mo Equipment
Using the New Hydrogen Attack Tendency Chart," PVP vol 239/MPC-vol 33, Serviceability of Petroleum Process and Power
Equipment, ASME, 1992.
- K. Kawano, "Recent Activities in High Temperature
Hydrogen Attack," to be presented in 2005.
- G. R. Prescott, "History and basis of Prediction of
Hydrogen Attack of C-1/2 Mo Steel," Material Property Conference, Vienna, Oct
19-21, 1994.
- A. S.
Birring, et al. "Method and Means for Detection of Hydrogen Attack by Ultrasonic
Wave Velocity Measurements" US Patent,
4,890,496, January 2, 1990
- A. S. Birring and K. Kawano,
"Ultrasonic Detection of Hydrogen Attack in Steels," Corrosion,
March, 1989.
- A. S. Birring, M. Riethmuller,
and K. Kawano, "Ultrasonic Techniques for Detection of High Temperature
Hydrogen Attack," Materials Evaluation, February, 2005.
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Highlights
NDE Associates
inspections detect HTHA in a reactor.

HTHA inspection of
a reactor in France.
The reactor was taken out of service after inspections found HTHA. |